The present invention relates to rolls, especially to rolls for use in calenders or analogous machines. More particularly the invention relates to improvements in systems for heating the rotary envelopes or shells of calender rolls or the like. Still more particularly, the invention relates to improvements in systems for electromagnetic heating of shells which form part of calender rolls or the like.
It is already known to heat the ferromagnetic shell of a calender roll by resorting to a set of magnets, especially electromagnets, whose pole faces are adjacent to a surface of the shell and which serve to induce currents in the material of the shell.
German publication entitled "VDI-Bildungswerk BW 1407" discloses the results of systematic testing of a calender roll which is heated by inducing heating currents in its rotary shell. The tests involved placing the winding of an electromagnet around the shell of the calender roll and the utilization of a plurality of U-shaped magnetic yokes which were caused to overlie the winding and to extend in the axial direction of the shell. The shell constituted an annular conductor for the heating current. The heating action was not satisfactory so that the aforementioned publication already contains a proposal to replace the magnetic heating system with a system which employs a circulating hydraulic fluid.
Heating systems which rely on a circulating hydraulic fluid exhibit the drawback that they cannot ensure the maintenance of a constant temperature or of a predictable pattern of temperatures along the periphery of the shell, i.e., in the region where the shell of a calender roll acts upon a running web of paper, textile or a like material. The main reason for such inability of a hydraulic heating system to ensure the establishment and maintenance of a constant temperature of a predictable pattern of temperatures along the external surface of the shell is that the temperature of the fluid changes during flow through the roll. Attempts to uniformize the temperature of the conveyed fluid, as considered in the axial direction of the shell, have met with limited success. Moreover, such uniformizing techniques are satisfactory only when the temperature of the external surface is to be maintained within a narrow band of the full range of desirable or required temperatures.